Acrylonitrile (ACN) is a large-scale industrial product which is manufactured almost exclusively by ammoxidation of propene in accordance with the following equation: EQU CH.sub.2 .dbd.CH--CH.sub.3 +NH.sub.3 +1.sup.1/2 O.sub.2 .fwdarw.CH.sub.2 .dbd.CH--CN+3H.sub.2 O
Theoretically, about one ton of water is also formed per ton of ACN. Since the selectivity for the desired ACN is not 100%, the quantity of water is greater and amounts to about 1.5 tons or more per ton of ACN. This effluent is loaded with undesired by-products, which include nicotinonitrile (molecular weight MW=104), fumaronitrile (MW=78), succinonitrile (MW=80), 3-picoline (MW=93) and 1-H-pyrazole (MW=68), and traces of acrolein.
Acrylonitrile production includes a purification process, at the end of the synthesis. The purification section of an acrylonitrile plant consists of several distillation columns, including a recovery column, a heads column, a drying column and a product column. Even though several distillation steps are involved, traces of contaminating acrolein may still be present in the acrylonitrile product.
One of the reasons that acrolein impurities in acrylonitrile are so detrimental is that acrylonitrile produced may be utilized to make acrylamide. Acrylamide is a common monomer useful for polymerization. Aldehyde impurities such as acrolein can be harmful to the polymerization because they may act as cross-linking agents. Acrolein, which has been identified as an impurity in acrylonitrile at levels from about 2 to about 20 parts per million (ppm), may act as a cross-linker itself or as an alkylol adduct with acrylamide. The addition of measured quantities of acrolein to aqueous solutions of crystalline acrylamide has demonstrated the strong effect of this aldehyde and its adduct with acrylamide on polymers formed from the polymerization of acrylamide. Even at the very low impurity levels of less than 20 parts per million of acrolein in acrylamide, polymers having an unsatisfactorily reduced molecular weight are formed. Consequently, such polymers formed from impure acrylamide which is in turn the result of impure acrylonitrile, are rendered commercially unsatisfactory for most water treatment applications wherein polymeric flocculants would be useful.
Accordingly, several methods for reducing or removing acrolein from acrylonitrile to augment the distillation purification process have been proposed. Japanese Patent Application 53-60040 (Publication No.54-151915) of Mitsui Toatsu Chemicals, Inc. discloses the use of a porous-type ion exchange resin which possesses a primary and/or secondary amine to remove acrolein from acrylonitrile. That patent application compares the use of a porous type free base resin, available under the name Diaion WA 20, with a gel type free base resin, available under the name Amberlite IRA-45, and concludes that the gel type cannot be used to remove acrolein. Thus, acrylonitrile containing 2.5 ppm acrolein is said to be purified to a level below the detection limit (0.1 ppm) with the porous type resin while still containing 2.1 to 2.3 ppm after treatment with the gel type resin. Japanese Patent Application 75/139,341 (Publication No. 77-68118) of Yoshiaki Ito et al. discloses stirring acrylonitrile containing acrolein for three hours with acetyl acetone, dimethylaminoethanol and 10 percent aqueous ferric chloride solution followed by distillation to give pure acrylonitrile. Cationic exchange resins coated with polyamine have been disclosed for nitrile purification in EP 0814 076 Al. Acrolein removal by contacting contaminated acrylonitrile with a gel type weak base ion exchanger having primary and/or secondary amine functionality is disclosed in GB 2 114 118A. However, these techniques are disadvantageous in that for each of these instances, another type of column would need to be added to the purification section, requiring reconfiguring of the plant set-up.
In U.S. Pat. No. 3,876,508, pH adjustment within the distillation column is disclosed as a technique to remove acrylonitrile from the ammonium salt byproducts of the acrylonitrile process. Extractive distillation of acrylonitrile in the presence of polyhydroxy compounds, by adding such compounds to the distillation column to remove contaminants such as acrolein has been disclosed in U.S. Pat. No. 3,257,446.
However, it is evident then that there remains a need for improved methods for removing aldehyde impurities in acrylonitrile so that purer acrylamide may be formed therefrom, to improve the quality of polymers made from the purer acrylamide.